A vehicle that uses electricity as driving energy usually employs a high-voltage (for example, 200 V) DC power source configured as an ungrounded power source insulated from a vehicle body. To detect a ground fault or an insulated condition of the ungrounded power source with respect to the vehicle body, i.e., a ground potential portion, a detecting unit employs a flying capacitor.
The detecting unit controls internal switches to charge the flying capacitor with a charge amount corresponding to a voltage of the DC power source insulated from the ground potential portion, a charge amount corresponding to a ground fault resistance on the positive terminal side of the DC power source, and a charge amount corresponding to a ground fault resistance on the negative terminal side of the DC power source. Thereafter, a controller of the detecting unit measures a charge voltage at each of the charge amounts, calculates the ground fault resistances on the positive and negative terminal sides of the DC power source, and detects a ground fault or an insulated condition of the DC power source. A related document is, for example, Japanese Unexamined Patent Application Publication NO. 2007-170983.